1. Field of the Invention
The present invention relates to a glow plug for previously heating a diesel engine or the like, a spark plug for an internal combustion engine, and a manufacturing method therefor.
2. Description of the Related Art
In general, a glow plug has a structure that a resistance heater is disposed in a main metal shell having the outer surface on which a joining thread portion has been formed such that a leading end heating portion of the resistance heater projects over either end surface of the main metal shell. The thread portion is used to join the glow plug to an engine head.
A spark plug for igniting a gasoline engine for an automobile or the like incorporates an insulating member disposed on the outside of a central electrode and a main metal shell disposed on the outside of the insulating member. Moreover, a ground electrode for forming a spark discharge gap from the central electrode is joined to the main metal shell. A joining thread portion provided for the outer surface of the main metal shell is used to joint the spark plug to the cylinder head of the engine.
The main metal shell is usually made of an iron material, such as carbon steel, and structured to have the surface applied with zinc plating to prevent corrosion. Although the zinc-plated layer has an excellent anti-corrosion effect for iron, the zinc-plated layer formed on iron can easily be consumed owing to sacrificial corrosion as known. What is worse, the zinc-plated layer is decolored to white owing to zinc oxide, causing the quality of the appearance to deteriorate. Therefore, a major portion of the glow plugs and the spark plug is structured such that the surface of the zinc-plated layer is coated with a chromate film to prevent corrosion of the plated layer.
The chromate film to be formed on the main metal shell of the glow plug and the spark plug has been a so-called yellow chromate film. Since the yellow chromate film exhibits excellent anti-corrosion performance, the yellow chromate film is widely employed in a variety of fields including coating of the inner surface of a can as well as the glow plug and the spark plug. Since a portion of contained chrome components is hexavalent chrome, use of the yellow chromate film has gradually been inhibited in recent years owing to global focusing on the environmental protection. For example, discontinuance of the chromate film containing hexavalent chrome in the future has been considered in, for example, the automobile industrial field in which glow plugs and spark plugs are used in a large quantity. Since a processing bath for forming the yellow chromate film contains hexavalent chrome at a relatively high concentration, there arises a problem in that an excessively large cost is required to dispose waste water.
Therefore, chromate films of a type which does not contain hexavalent chrome, that is, films of a type that the substantially overall portion of chrome components is contained as trivalent chrome have been researched and developed at a relatively earlier time. Thus, processing baths containing hexavalent chrome at a relatively low concentration or baths containing no hexavalent chrome have been developed. Therefore, the problem of disposal of waste waster has been overcome. However, the chromate film employing the trivalent chrome suffers from unsatisfactory anti-corrosion performance as compared with the yellow chromate film. Therefore, wide use of the yellow chromate film as a film with which the main metal shell of the glow plug and the spark plug is coated has not been realized.
Further, the conventional chromate films including the yellow chromate films suffer from a common problem of unsatisfactory heat resistance. Since, for example, the engine of an automobile incorporates a cylinder head to which the spark plug is joined is cooled with water, the temperature of the spark plug is not raised excessively. When the operation of the engine is continued under a condition that a great load of heat is exerted or when the spark plug is joined relatively adjacent to the exhaust manifold, the temperature of the main metal shell is sometimes raised to about 200° C. to 300° C. In the foregoing case, the chromate film easily deteriorates. Thus, there arises a problem in that the anti-corrosion performance rapidly deteriorates. Moreover, the conventional chromate film suffers from deterioration in the performance owing to attack of an acid component, such as carbon dioxide, a nitrogen oxide or a sulfur oxide, contained in acid rain and exhaust gas and, in a case of a gas engine, acid water produced by the engine.